I think this is an internal thing of the GraphQL feature. It deals differently from Dgraph. But you can add it manually via alter endpoint or bulk edit on Ratel’s schema panel.
Before doing something, check what the type it would be. Not sure what Dgraph creates in the schema related to graphql’s interface. Once you know it, you can add
e.g:
type Post { #this is just an example.
<~post.saved>
}